High-performance ZnO thin film transistors with sputtering SiO2/Ta2O5/SiO2 multilayer gate dielectric

A high-performance ZnO thin film transistor （ZnO-TFT） with SiO2/Ta2O5/SiO2 (STS) multilayer gate insulator is fabricated by sputtering at room temperature. Compared to ZnO-TFTs with sputtering SiO2 gate insulator, its electrical characteristics are significantly improved, such as the field effect mobility enhanced from 11.2 to 52.4 cm2/V s, threshold voltage decreased from 4.2 to 2 V, and sub-threshold swing improved from 0.61 to 0.28 V/dec. The improvements are attributed to the high gate capacitance (from 50 to 150 nF/cm2) as well as nice surface morphology by using dielectric with high～k Ta2O5 sandwiched by SiO2 layers. The capacitance–voltage characteristic of a metal–insulator–semiconductor capacitor with the structure of Indium Tin Oxide/STS/ZnO/Al was investigated and the trap charges at the interface or bulk is evaluated to be 2.24 × 1012 cm−2. From the slope of C−2 versus gate voltage, the doping density ND of ZnO is estimated to be 1.49 × 1016 cm−3.